In a typical mixer-settler, in the first step, the aqueous and organic phases are pumped into a mixer or mixers in order to achieve a uniform liquid-liquid dispersion and a small droplet size. In the VSF® technology (stands for Vertical Smooth Flow) developed by the applicant this first step is performed in a pump-mixer called Dispersion Overflow Pump (DOP( ) (disclosed e.g. in document U.S. Pat. No. 5,662,871) and in a set of two SPIROK® helical mixers (disclosed in e.g. document U.S. Pat. No. 5,185,081). After mixing, the dispersion is fed into a settler. The settler is typically a large tank which is square in plan and its square area is about several hundred square meters. Dispersion is fed into the settler at the front end of the settler. A distributor fence is arranged at the feed end of the settler to distribute the flow of the dispersion to the whole width of the settler. In the settler, the dispersion moves towards the settler back wall and, at the same time, the phases separate by gravity into two layers with a dispersion band remaining between them. Typically, separation fences are arranged in the settler tank to enhance coalescence of the dispersion. In the VSF® technology the separation fences are so-called DDG® fences (Dispersion Depletor Gate) (disclosed e.g. in document U.S. Pat. No. 7,517,461).
At the rear end of the settler, an adjustable weir and launders are used to control the vertical position of the phase interface and to collect and discharge both phases, respectively. Arrangements of settlers and launders are disclosed also e.g. in documents WO 97/40899, WO 97/40900, WO 97/40901, WO 2009/063128 A1 and WO 2010/097516 A1.
The known launder typically comprises two launders arranged in parallel side-by-side. One of the launders is an overflow launder arranged to receive the lighter solution (e.g. organic phase) as an overflow from the settler and the other launder is an underflow launder arranged to receive the heavier solution (e.g. aqueous solution) as an underflow from the settler. The launder arrangement is made of a fiber-reinforced plastic composite by hand laminating, or by filament winding as described in WO 2010/097516 A1. WO 2009/063128 discloses that the whole launder is manufactured at a place of manufacture, such as in an engineering workshop, into a self-supporting subassembly which is transferred as a uniform entity to the site of installation where it is installed on the bottom of the settler.
So far, a solvent extraction plant including the launder has been project specified. In each case the layout of the plant and the equipment have been unique. There has not been a possibility for the productization of launders. The present launders have nonstandard transport dimensions requiring oversize transport which is expensive. Launders known in the prior art also require most of the construction work to be done at the site. This causes problems because of the crucial influence of local factors. It may be difficult to get local suppliers. It has been difficult to control the quality of the site work by local suppliers. Further, the maintenance of the present launders requires a long downtime of the whole solvent extraction settler with which the launder requiring maintenance is connected.